Ultrashort Echo Time (UTE) 1H Imaging
Lung imaging via 1H MRI is intrinsically difficult due to multiple air-tissue interfaces within the lungs causing local gradients and severe magnetic field susceptibility, which leads to an exceedingly short effective transverse relaxation time (T2*). Additionally, the lungs have low proton density, resulting in low signal to noise ratio, and the physiological motion caused by respiration and cardiac pulsation further reduces lung signal. Until recently the application of MRI in lung imaging has lagged behind other organs. The developments of more powerful hardware along with faster MRI techniques have enabled detailed noninvasive imaging of pulmonary tissues. Several lung MRI studies were conducted using spin-echo times (TE) ranging from 0.8 – 4.2 milliseconds and demonstrated that the potential to evaluate lung parenchymal tissues increases with decreasing TE. Ultrashort TE (UTE) MRI in conjunction with projection acquisition of the free inducting decay allows for the reduction of the TE to less than 100 microseconds and minimizes the signal decay due to short T2*. This technique has been proven as a viable method for imaging both rat and human parenchyma tissue in finer detail compared to conventional MRI. Yet, due to need for special modifications of pulse sequences and reconstruction algorithms the application of UTE MRI in imaging lungs is relatively sparse.
Hyperpolarized 3He Imaging
Hyperpolarized (HP) 3Helium has been used in research as an inhaled contrast agent to improve image quality for anatomic and functional MRI of the lungs. Over the past two decades inhaled Hp-3He repeatedly has been demonstrated as a safe and highly tolerable form of pulmonary MRI contrast in a wide range of subjects—including infants, children, and adults, participating as healthy volunteers and as those with a variety of respiratory conditions ranging in severity. With Hp-3He MRI, we are able to image temporal distribution of ventilation, assess air space size, and visualize lung microstructures with high image quality
Hyperpolarized 129Xe Imaging
Hyperpolarized (HP) 129Xenon imaging of the lungs is akin to HP-3Helium imaging in that the inhaled gas itself provides the signal, but xenon possesses a unique property that small amounts of the gas can dissolve into the tissue and blood. However, dissolved phase xenon also exhibits a short T2*, similar to the lung parenchymal tissues, requiring the use of UTE image sequences, as mentioned for 1H imaging. The dissolved xenon so happens to be chemically shifted from the gaseous xenon thus providing a means of imaging and quantifying the process of perfusion within the lung. By imaging both ventilation (gaseous xenon) and perfusion (dissolved xenon), we can map out impaired gas exchange to the blood. The ability to locate regions of impaired gas exchange, without ionizing radiation, provides much more information for care.